The invention relates generally to ball bearing assemblies suitable for heavy duty use, for example, in connection with the turntables of cranes, power shovels and the like, and more particularly, to spacers interposed between the balls of such ball bearing assemblies.
Ball bearing assemblies for equipment of the character referred to, encounter eccentric vertical and axial loads which set up high and low load zones in the bearing. Bearing balls are found to alternately crowd and spread apart in these high and low load zones, respectively. Such crowding of the balls under extreme loads creates substantial compressive forces on the balls and any ball spacers interposed therebetween. Ball spacers or separators are necessary in these bearings to prevent oppositely moving surfaces of adjacent balls from coming into contact with each other since the rubbing velocity of the ball surfaces is twice that of the velocity of the ball race. Also, rubbing pressures between the balls are considerable when the balls crowd together in a heavily loaded section of the ball bearing. When a resiliently deformable low coefficient of friction plastic material such as nylon is employed as a ball spacer, extreme crowding and compressive force causes the spacers to radially crack due to excessive circumferential or hoop stress.
Various forms of reinforced resiliently deformable spacers have been interposed between the balls of a ball bearing assembly to prevent the balls from coming into direct frictional contact. The simplest approach involves making the body of the spacer wider so that the spacer will withstand greater stresses. However, this approach reduces the load or capacity of the bearing assembly since fewer rolling elements may be interposed in a bearing assembly of a given diameter when such spacers are present. This penalty in capacity becomes important as safety factors are reduced and users of the bearings become size conscious in an evermore competitive industry.
Another approach involves the use of a spacer formed from a resilient deformable plastic body portion having a surrounding metal band. Generally, these banded spacers have body portions that are thermoplastically molded into the circumferential band. This construction allows the molded body portions to shrink away from the reinforcing metal bands upon cooling to ambient temperatures after the molding operation. Thus, such spacers derive little or no benefit from the strength of the bands.
In another type of banded spacer, a ball bearing spacer is provided with a resiliently deformable body portion having a circumferential band press fitted on the cylindrical periphery of the spacer whereby the body portion is subjected to an initial compressive prestress and the band is subjected to an initial tensile prestress. By prestressing or preloading the components of the spacer, the body portion thereof is materially increased in strength and rupture or cracking thereof due to excessive hoop stress is substantially eliminated. Because of the increased strength of this type of spacer construction, the thickness of the spacer can be substantially reduced over that of unreinforced spacer constructions. However, the tolerances between the cylindrical body portion and the surrounding band are critical and the cylindrical spacer must be precision ground to a specific size before being press fit into the band to create the desirable initial compressive prestress. This operation, of course, involves a substantial amount of labor and expense.
In another type of ball bearing spacer, a reinforcing metal band or spring-like structure is embedded in a generally cylindrical molded body portion. However, in this spacer construction, no attempt is made to create an initial compressive prestress on the cylindrical body portion of the spacer. This is clear, especially in the case where a serpentine or spring-like internal reinforcement member is employed, since such an open-ended structure could not support an initial prestress.